Aquaporins are water channel proteins present in the membranes of plants and animal cells and belong to a family of proteins known as the MIP (membrane intrinsic protein) family. Maurel et al, J. Exp. Botany, 48: Special Issue, 421-430, 1997. These proteins exhibit a typical structure of 24-28 kDa with six predicted transmembrane spanning domains and some perfectly conserved residues including an Asn-Pro-Ala motif, in both the first intra-cytoplasmic and third extracytoplasmic loops of the protein. Reizer et al., Critical Reviews in Biochemistry and Molecular Biology, 28: 235-57, 1993. Within the plant cell, aquaporins are found in both the vacuolar and plasma membranes.
Aquaporins are involved in: cell expansion and cell volume regulation, transcellular water flow, adjustments to water deficit, regulated water delivery, and adaptation to cold stress by osmotic adjustments. Some aquaporins have been shown to be desiccation and salt stress induced in plants. Yamaguchi-Shinozaki et al., Plant Cell Physiol., 33: 217-224, 1992; Guerrero et al., Plant Mol. Biol., 15: 11-26, 1990. Kaldenhoff et al., (Plant J., 7: 87-95, (1995)) showed that the down-regulation of aquaporins in Arabidopsis plants that express antisense constructs results in a slower swelling and bursting of the isolated protoplasts.
Tonoplast Intrinsic Proteins (TIP) are MIP homologs that have been identified in seeds from a wide range of monocot and dicot species. Johnson et al., Plant Physiology, 91: 1006-13, 1989. Full or partial sequences have been determined in bean (Johnson et al., Plant Cell, 2: 525-32, 1990), Arabidopsis thaliana (Hofte et al., Plant Physiology, 99: 561-70, 1992), pumpkin (Cucurbita sp.) (Inoue et al., Plant Molecular Biology, 28: 1089-1101, 1995), and Norway spruce (Picea abies) (Oliviusson and Hakman, Physiologia Plantarum, 95: 288-95, 1995) and confirm the high conservation of these proteins in seeds. Full or partial sequences of more than 20 MIP homologs in Arabidopsis are reported to be available in the sequence data banks. Maurel et al., J. Exp. Botany, 48: Special Issue, 421-430, 1997.
An important parameter of crop plants is water use efficiency. Plants able to make better use of scarce water resources promise to make agriculture more sustainable in many areas of the U.S. and the world. Accordingly, what is needed in the art are plants which are desiccation-, salt-, cold-, or drought-tolerant. The present invention provides this and other advantages.